1. Field of the Invention
The invention relates generally to resistivity logging tools. More particularly, the invention relates to tools and methods of resistivity measurements in wells drilled with oil-based muds.
2. Background Art
In order to minimize the effect of water coning, the number of high angle (deviated) and horizontal wells drilled in small and complex reservoirs has increased significantly over the past years. Many of these reservoirs comprise a series of small targets that on their own would not be economical to drill. To make drilling such reservoirs economical, the industry has developed directed drilling techniques to drill deviated or horizontal wells.
Many tools for drilling deviated or horizontal wells are available. Examples include the geosteering tool sold under the trade name of GST™ by Schlumberger Technology Corporation (Houston, Tex.). The GST™ tool provides azimuthal resistivity measurements close to the drilling bit. The azimuthal resistivity measurements can be used to steer the drill bit to follow a path to the hydrocarbon zones and stay away from water zones. The GST™ tool can also determine whether the well path is getting out of the pay zone.
U.S. Pat. No. 5,235,285 issued to Clark et al. and assigned to the assignee of the present invention discloses a tool that measures the resistivity at the bit. Examples of tools based on this and related principles include one sold under the trade name of RAB™ (resistivity at the bit) and another sold under the trade name of GVR™ (geovision resistivity) by Schlumberger Technology Corporation (Houston, Tex.). These tools are capable of delivering full borehole resistivity images of the reservoir rock being drilled. This capability makes it possible to detect small geological structures or thin formation layers while drilling and allows one to image reservoir structural and stratigraphic dips. Being able to detect and visualize the well path while drilling is crucial in placing the well in the proper location—to stay within the pay zone and to avoid crossing the boundary.
Unfortunately, these tools, GST™, RAB™, and GVR™, are limited to wells drilled with water-based mud (WBM), while most high-angle and horizontal wells are drilled with oil-based mud (OBM). All these tools, GST™, RAB™ and GVR™, are based on low frequency electrode devices that generally do not operate in wells drilled with OBM.
In the past, in order to measure formation dips in wells drilled with OBM, some tools (e.g., a standard dipmeter) have been equipped with “scratchers” to remove resistive mud cakes in order to improve electrical contacts between the rock formation and the electrodes.
The first device developed specifically for the measurements of formation dips in wells drilled with OBM was an OBM dipmeter based on capacitive coupling. One example of such an OBM dipmeter is disclosed in U.S. Pat. No. 3,973,181 issued to Calvert and assigned to the assignee of the present invention.
This device operates at high frequency (10 MHz) to minimize the effect of standoffs. A single guarded (insulated) button was mounted on each of the four pads of a standard dipmeter. Thus, this tool can image four sectors of the borehole; however, it does not have enough coverage of the borehole to provide full borehole images.
Later, a new sensor, an OBM dipmeter, was disclosed in U.S. Pat. No. 4,780,678 issued to Kleinberg et al. and assigned to the assignee of the present invention. This new OBM dipmeter operates at a frequency of around 50 MHz. It uses a differential induction coil system that responds primarily to the contrast in resistivity between adjacent layers. As such, this tool has the capability to measure formation dip in OBM, but it has no imaging capability. A tool based on this technique is sold under the trade name of OBDT™ by Schlumberger Technology Corporation (Houston, Tex.).
More recently, an Oil Base Mud Imager was disclosed in U.S. Pat. No. 6,191,588 B1 issued to Chen and assigned to the assignee of the present invention. A tool based on this techniques is sold by Schlumberger under the trade name of OBMI™. This tool provides for the first time a true borehole image in wells drilled with OBM. A similar resistivity tool that can be used to image a well drilled with OBM is disclosed in U.S. Pat. No. 6,600,321 B2 issued to Evans. This tool is capable of providing wellbore resistivity determinations and imaging based on capacitive coupling.
All the tools described above are wireline tools. While progress has been made in the development of wireline OBM tools, the development of logging-while-drilling (LWD) or measurement-while-drilling (MWD) OBM resistivity tools has been relatively slower. The only LWD/MWD tool capable of providing images of wells drilled with OBM is the azimuthal density neutron tool sold under the trade name of ADN™ by Schlumberger Technology Corporation (Houston, Tex.). However, ADN™ can only provide a 16-sector density image. Moreover, the densities of typical rock formations have a more limited range, typically 2 to 3 g/cc, as compared to the range of resistivities, typically 0.2 to 2000 ohm-meter. Therefore, an imaging tool based on resistivity is more desirable.
Furthermore, a logging sensor used on an LWD tool may not be able to maintain contact with the borehole wall at all times. Tool standoffs may reduce the accuracy of the measurements. Therefore, it is desirable that a sensor to be used on an LWD tool have the ability to minimize or eliminate the standoffs.